1. Field of the Invention
The present invention relates to a radiation source, a lithographic apparatus, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. In a lithographic apparatus as described above a device for generating radiation or radiation source will be present.
In a lithographic apparatus the size of features that can be imaged onto a substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet, also referred to as XUV or EUV, radiation. The abbreviation ‘XUV’ generally refers to the wavelength range from several tenths of a nanometer to several tens of nanometers, combining the soft x-ray and vacuum UV range, whereas the term ‘EUV’ is normally used in conjunction with lithography (EVL) and refers to a radiation band from approximately 5 to 20 mn, i.e. part of the XUV range.
Two main types of XUV radiation generating devices or sources are currently being pursued, a laser-produced plasma (LPP) and a discharge-produced plasma (DPP). In an LPP source, one or more pulsed laser beams are typically focused on a jet of liquid or solid to create a plasma that emits the desired radiation. The jet is typically created by forcing a suitable material at high speed through a nozzle. Such a device is described in U.S. Pat. No. 6,002,744, which disloses an LPP EUV source including a vacuum chamber into which a jet of liquid is injected using a nozzle.
In general, LPP sources have several advantages compared to DPP sources. In LPP sources, the distances between the hot plasma and the source surfaces are relatively large, reducing damage to the source components and thus reducing debris production. The distances between the hot plasma and the source surfaces are relatively large, reducing the heating of these surfaces, which in turn reduces the need for cooling and reduces the amount of infra-red radiation emitted by the source. The relatively open geometry of the construction allows radiation to be collected over a wide range of angles, increasing the efficiency of the source.
In contrast, a DPP source generates plasma by a discharge in a substance, for example a gas or vapor, between an anode and a cathode, and may subsequently create a high-temperature discharge plasma by Ohmic heating caused by a pulsed current flowing through the plasma. In this case, the desired radiation is emitted by the high-temperature discharge plasma. Such a device is described in European Patent Application 03255825.6, filed Sep. 17, 2003 in the name of the applicant. This application describes a radiation source providing radiation in the EUV range of the electromagnetic spectrum (i.e. of 5–20 nm wavelength). The radiation source includes several plasma discharge elements, and each element includes a cathode and an anode. During operation, the EUV radiation is generated by creating a pinch as described in FIGS. 5A to 5E of EP 03255825.6. The application discloses the triggering of the pinch using an electric potential and/or irradiating a laser beam on a suitable surface. The laser used has typically a lower power than the laser(s) used in an LPP source.
In general, however, DPP sources have several advantages compared to LPP sources. In DPP sources, the efficiency of the source is higher, approximately 0.5% for a DPP compared to 0.05% for an LPP. DPP sources also have a lower cost and require fewer, less expensive part replacements.